Hydraulic borehole mining for pitching coal seams. Final report. [Double drill method]

This project was undertaken to extend the vertical borehole mining concept to the new tool configurations needed to economically mine pitching coal seams. The objective was to determine the optimum borehole mining plan and system for mining coal seams less than 10 feet thick that pitch more than 25 degrees. Of the methods considered, the double-drill method was selected for further study. This method uses a larger vertical hole for the pumping tool and a smaller slant hole for the cutting tool. The slanted cutting hole is drilled down-the-dip on the footwall of the seam to intersect the vertical hole at maximum depth. The cuttings thus drain down the cutting hole to its intersection with the pumping hole, from which they are pumped to the surface. Detailed engineering efforts demonstrated double-drill method technical feasibility and performance capabilities. However, these efforts also revealed two items that require further development: (1) the borehole slurry pump and (2) the means to ensure vertical and slant borehole intersection. Recommendations as to the type of equipments needed and the effort required to develop them for this application are given. Detailed economic studies of the double-drill method were conducted. These studies indicated that this method hasmore » a very favorable economic potential, i.e., potential internal rates of return of about 50 percent per year.This implies that pitching coal seams could be mined more profitably using this method than can level coal seams using conventional underground mining method. Based on the success of this project, it is expected that double-drill hydraulic borehole mining will prove to be a valuable tool for recovering pitching coal resources if its development is continued.« less